Lately, there have been a growing number of opportunities where the inkjet recording method is utilized for the purpose of producing large outdoor advertisements, etc., that require a wide image area. Base materials used for these large outdoor advertisements include polyvinyl chloride sheets having strength to withstand outdoor use, such as sheets made solely of polyvinyl chloride resin, as well as tarpaulin and other sheets made of composite materials. As inkjet printing inks used for printing on these polyvinyl chloride sheets, photocurable ink compositions for inkjet printing that do not use volatile components are proposed.
For the aforementioned outdoor advertisements, etc., to withstand weather, not only must their base material be tough, but the ink composition must also be able to form a tough film. However, photocurable materials that can form a tough film tend to have lower adhesion to the base materials because the films are generally very hard. For an ink composition of the aforementioned type, therefore, sometimes a method is adopted whereby a component that dissolves the base material is blended into the ink so as to improve its adhesion to the base material. For example, an active energy ray-curable inkjet ink that contains, at a specific ratio, a compound containing ethylene double bonds that does not dissolve polyvinyl chloride, and a component containing ethylene double bonds (N-vinyl caprolactam) that dissolves polyvinyl chloride is proposed for use on a polyvinyl chloride sheet (refer to Patent Literature 1).
However, use of such inkjet ink to manufacture printed matter presents a problem in that the printed matter cockles easily (cockling refers to deflection of the base material), although the ink adheres to a polyvinyl chloride sheet favorably.
In addition, when images are cured using a metal halide lamp or other conventional, generally used high-energy light source, problems also occur such as generation of ozone, need to increase the size of the irradiating equipment, shorter service life of the lamp, etc. Accordingly, low-energy light-emitting diode lamps (LED lamps) are used as light sources in recent years. This gives rise to such requirements as curability under LED lamps, printing performance (adhesion, improvement of cockling, etc.), and safety (flash point, irritability, etc.), and photocurable ink compositions for inkjet printing that can meet these requirements are proposed.
Among such photocurable ink compositions for inkjet printing that have been proposed are, for example: a photocurable ink composition for inkjet printing that contains pigment, benzyl acrylate, N-vinyl caprolactam, acrylated amine compound having two photopolymerizable function groups and two amino groups in its molecule, and sensitizer (refer to Patent Literature 2); and a photocurable ink composition for inkjet printing that contains pigment, specified amount of a compound having (meth)acrylate and vinyl ether groups in its molecule, and specified amount of vinyl caprolactam (refer to Patent Literature 3).